This invention relates to alarm devices in general, and more particularly to an improved alarm device which serves the dual function of a time alarm and a smoke alarm.
For the past few years, there have appeared on the market numerous smoke alarm devices. These devices can be classified according to the type of power they use and according to the type detector which is used. Basically such devices are available either as house current operated devices or battery operated devices. The advantage of devices operating on house current is that one does not have to be concerned about batteries going dead. The advantage of the battery operated devices, of course, is that they continue to operate even when there is a loss of house current due to a fire or for some other reason. The detectors used on these alarms are normally either a photoelectric type detector which includes a light source and a photoreceptor or an ionization detector. As used herein, the term "smoke detector" is generic to those two types of detectors.
Although various possibilities exist, one of the most efficient type photoelectric devices comprises a light emitting diode as the light source and a photodiode as the photo-receptor. The use of a light emitting diode makes a battery operated photoelectric detector much more feasible. In photoelectric detectors the two devices are not lined up and the receptor normally does not intercept the light. Smoke particles entering the chamber scatter the light causing some of it to be reflected onto the receptor. When the receptor receives sufficient light an alarm is sounded.
Ionization detectors, on the other hand, work on the principal of bombarding the air in a small chamber with radioactive particles. The radioactive particles ionize the air so that it will carry a weak electric current. When the particles released due to combustion enter the chamber they interfere with the current flow and, at a predetermined level, the decrease in current flow causes an alarm to trigger. Tests have shown that ionization detectors respond more quickly to flaming fires and photoelectric units respond much faster to smoldering fires. There has been much discussion over which of these are the best and what the detection limits should be.
These smoke alarms sound quite loudly, since they are normally placed in a central location in a home and must wake up the whole family who may be in different bedrooms scattered about the house. Normally the best place to locate such detectors is in an upstairs hall where heat and smoke would reach the detector from anywhere in the house. Thus, with presently available devices it is possible to protect oneself from most home fires. However, there are other instances where detection is not so available.
Suppose a detector is located in the hall outside the bedrooms and the family goes to sleep with their bedroom doors closed. If a fire starts in one of the bedrooms it is quite possible that the alarm will not sound soon enough to allow the occupants of that bedroom to escape safely since the smoke must essentially fill the room and then begin to seep out before it reaches the detector in the hall. One solution to this would be, of course, to supply detectors in each room. However, these detectors are fairly bulky and expensive. A traveller faces a more serious problem. Although most hotels and motels are equipped with fire protection systems, typically sprinkler systems, it is well known that many people have been killed or injured in hotel fires. One reason for this may be that the sprinkler systems, which normally respond to heat, in the case of a smoldering fire, may not respond until it is too late. Furthermore, in various parts of the world standards are not always as high as they are in the more developed countries and some hotels and motels may be without fire protection.
Of course, the solution to this problem might be carrying a smoke detector along when travelling. Although, this would work, one wonders what would happen if the smoke detector was accidentally triggered due to smoke from a cigarette, for example. The alarm has a particularly loud and shrill sound which would undoubtedly disturb all of the guests in the surrounding rooms and which could eventually lead to the banning of such detectors in hotels.
It thus becomes apparent that there is a need for additional protection against smoke and fire, particularly for the traveller.